1. Field of the Invention
The present invention relates, in general, to an optical pickup device and, more particularly, to an optical pickup device, which can prevent noise, generated from other layers, from flowing into servo signals.
2. Description of the Related Art
Ever since a Compact Disc (CD), which is an optical storage medium capable of storing up to 74 minutes of audio (music) or 650 Mbytes of data has been marketed, a Digital Versatile Disc (DVD) capable of holding two hours of Standard Definition (SD)-level video has been widely commercialized. Further, a Blu-ray Disc (BD) or a High-Definition (HD) DVD, capable of storing HD-level movie, will be introduced to the market in the near future.
Optical storage media, such as a CD, a DVD, and a BD, are disc-type media in which data is stored using optical characteristics, and data is recorded on an optical disc or the data recorded on the disc is reproduced through an optical pickup device. Optical storage media include discs for reproduction, on which data was previously recorded, and discs for recording which enable writing or rewriting, such as a CD-Recordable/Rewritable(R/RW), DVD-R/+R/-RW/+RW/Random Access Memory (RAM), a BD-Recordable/Recordable-Rewritable (-R/-RE).
An optical pickup records data on a disc or reads data from the optical disc in a state in which a laser beam is accurately focused on the track of the disc, which is rotating at high speed.
In order to obtain servo signals, such as a focusing error signal and a tracking error signal, corresponding to the position error of a beam spot focused on a disc, and to correct the position error of the beam spot on the basis of the servo signals, that is, in order to perform a servo operation, an optical pickup is implemented such that optical parts, such as an objective lens and a beam splitter, mechanical parts, such as an actuator and a base, and electrical parts, such as a laser diode and a photodetector, are arranged therein.
An astigmatic method is generally used for the detection of a focusing error signal, regardless of the type of disc and discs for recording/reproduction. In relation to the detection of a tracking error signal, a 3-beam method or a Differential Phase Detection (DPD) method is used in a disc for reproduction, and a Differential Push-Pull (DPP) method is representatively used in a disc for recording.
FIG. 1 illustrates the principles of the detection of a tracking error signal based on a DPP method.
The DPP method is a method which improves on a conventional 1-beam push-pull method and which can cancel offset occurring due to the movement of an objective lens in a radial direction or due to the tilt of a disc, and can detect a stable tracking error signal.
In the DPP method, a laser beam emitted from a light source is separated into three beams, that is, a 0th order diffracted beam and +/−1st order diffracted beams through a diffractive element called a grating. The grating is controlled such that, when a main beam, the 0th order diffracted beam, is arranged in the groove of a disc track, sub-beams, the +/−1st order diffracted beams, are arranged in lands adjacent to the groove in which the main beam is arranged (that is, such that the sub-beams are arranged to be spaced apart from the main beam by a distance of a ½ track pitch). Further, a tracking error signal is detected on the basis of differential signals of left and right signals of respective beams in a radial direction.
The main beam, reflected from the disc, is received by a 4-divided (a, b, c, d) main photodetector, and is detected as a Main Push-Pull (MPP)((A+D)−(B+C)) signal, which is a push-pull signal. Respective sub beams reflected from the disc are received by 2-divided (E1, E2) (F1, F2) sub photodetectors, and are detected as a Sub Push-Pull (SPP) ((E1−E2)+(F1−F2)) signal. When the sub beams are arranged to be spaced apart from the main beam by a distance of ½ track pitch, the phases of the MPP and SPP become opposite each other, as shown in FIG. 2.
Since offset occurs in the same direction both for MPP and SPP, according to the tilt or movement of an objective lens in a radial direction, an offset-free push-pull signal can be obtained if operation is performed using DPP=MPP−k×SPP (where k is a proportional constant). Further, a push-pull signal having greater amplitude can be obtained by subtracting the SPP having an opposite phase from the MPP.
Generally, the ratio of quantities of light of a main beam, used to generate an MPP signal, and sub beams, used to generate an SPP signal in the DPP method, is set to about 1:5:1 to 1:20:1, so that the light quantity of the sub beams is set to be equal to about ⅕ to 1/20 of that of the main beam. In the equation for obtaining the DPP signal, the proportional constant k is adjusted (for example, k is adjusted to 5 when the ratio of light quantities is 1:10:1), thus canceling the offset.
As described above, in order to use the DPP method, the angle of the sub beams, which are +/−1st order diffracted beams, must be adjusted while a grating is rotated. In this case, there is a disadvantage in that signal characteristics may be influenced by the extent of adjustment.
That is, when the main beam is positioned in a groove (or land), the sub beams must be positioned in the lands (or grooves). However, track pitches may differ from each other for respective disc types. For example, in the case of a DVD+RW or DVD-RW, the track pitch (Tp), indicating the distance between tracks, is 0.74 μm, and in the case of DVD-RAM, the track pitch is 0.615 μm, so that it is difficult to apply the same angle to discs having different track pitches.
Further, with the development of a BD or HD-DVD, the necessity for an optical disc recording/reproducing device capable of reproducing or recording all types of CD, DVD, and BD (or HD-DVD) has increased. However, since the numerical apertures of objective lenses required for the recording/reproduction of CD, DVD, and BD differ greatly from each other, it is almost impossible to reproduce all three types of discs using only a single objective lens.
In consideration of this necessity, an optical pickup, in which two objective lenses, that is, an objective lens for CD/DVD and an objective lens for BD or HD-DVD, are mounted on an actuator, has recently been developed. The two objective lenses can be arranged in the direction of the track (tangential direction) of a disc, or the inner/outer circumferential direction (radial direction) of the disc. When the objective lenses are arranged in the radial direction, it is not easy to access the innermost circumference of the disc, and thus an optical pickup in which two objective lenses are arranged in the track direction has been developed.
FIG. 3 illustrates an example in which one objective lens deviates from the central axis of a disc and is arranged off-axis when two objective lenses, mounted on a single actuator, are arranged in the track direction.
As shown in FIG. 3, when two objective lenses are arranged in the track direction, at least one of the two objective lenses deviates from the axis for connecting the inner and outer circumferences of a disc (axis passing through the center of the disc). In an optical system using the objective lens disposed at the location deviating from the central axis of the disc, since the relative positions of a main beam and sub beams required for DPP detection on the track (angle of the sub beams) vary as the optical pickup moves from the inner circumference of the disc to the outer circumference, the adjustment of the angle of the sub beams is meaningless.
Meanwhile, a multi-layer structure, in which two or more recording layers are formed to increase storage capacity, is being adopted in standards for DVD and BD. Further, it is expected that, even in the case of a high density disc, which will be developed in the future, a multi-layer structure will be generalized.
In order to increase the density of a disc, the wavelength of a laser beam is shortened, and the Numerical Aperture (NA) of an objective lens is gradually increased. In the case of a CD, a laser diode having a wavelength of 780 nm and an objective lens having an NA=0.45 are used. In the case of a DVD, a laser diode having a wavelength of 650 nm and an objective lens having an NA=0.6 are used. In the case of a BD, a laser diode having a wavelength of 405 nm and an objective lens having an NA=0.85 are used.
In a multi-layer disc, the interval between recording layers is determined to be approximately proportional to the focal depth of a beam spot. Since the focal depth is proportional to the wavelength of a laser beam and is inversely proportional to the square of the NA of an objective lens, the interval between recording layers must decrease as recording density increases.
When a recording or reproduction operation is performed on a multi-layer disc having a short interval between layers, a beam reflected from a layer adjacent to a current recording layer, that is, noise light from another layer, easily flows into photodetectors, as shown in FIG. 4.
Noise light from another layer also flows into a main photodetector for receiving a main beam and sub photodetectors for receiving sub beams, and thus influences servo signals as well as reproduced signals. In particular, the servo signals obtained using sub beams having a relatively small quantity of light are greatly influenced.
As shown in FIGS. 5A and 5B, there occurs a problem in that an SPP signal is excessively distorted due to noise light from another layer, and a DPP signal calculated based on the SPP signal, that is, a tracking error signal, is degraded. That is, when a disc having two or more recording layers is reproduced or recorded, noise occurs in reproduced signals and servo signals due to the beam reflected from another layer, as shown in FIGS. 4 and 5, and thus reproduction or recording performance may be deteriorated by this noise, and recording may occasionally be impossible.